Motor vehicle transmissions of the type addressed by this invention include several fluid operated torque transmitting devices referred herein as clutches, which are automatically engaged and disengaged according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission. The input shaft is coupled to an internal combustion engine through a fluid coupling such as a torque converter, and the output shaft is mechanically connected to drive one or more vehicle wheels.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios.
Shifting between the various speed ratios generally involves disengaging a clutch associated with the current or actual speed ratio Ract, and engaging a clutch associated with the desired speed ratio Rdes. The clutch to be released is referred to as the off-going clutch, while the clutch to be engaged is referred to as the on-coming clutch. Shifts of this type are referred to as clutch-to-clutch in that no speed responsive or freewheeling elements are used.
Shifting is initiated in response to a comparison between measured and predetermined values of one or more load condition parameters. The parameters typically include vehicle speed so that the transmission is successively upshifted to the upper speed ratios as the vehicle speed is increased and successively downshifted to the lower speed ratios as the vehicle speed is decreased.
The present invention is directed to the control of clutch-to-clutch downshifting during coast conditions--that is, where the vehicle speed is decreasing, with or without application of the service brakes, at closed or light engine throttle settings. When performing coast downshifts, one objective is to time the shift so as to minimize driveline disruption. This means that the speed differential across the on-coming clutch should be at or near zero at the time of clutch engagement. Another objective is to maintain the establishment of a speed ratio which will provide adequate performance in the event the operator terminates the coast condition by increasing the engine throttle setting. This means that successive shifting should occur as the vehicle slows to a stop and that the neutral intervals between disengagement of an off-going clutch and engagement of an on-coming clutch should be minimized.
The above-noted objectives were addressed by the clutch-to-clutch shift control method disclosed and claimed in the U.S. Pat. No. 4,671,139 to Downs et al., assigned to General Motors Corporation, and issued on June 9, 1987. According to that control method, the coast downshifts are initiated after the transmission input speed drops below the neutral idle speed of the engine. The off-going clutch is released to initiate a neutral interval during which the input speed returns to the engine neutral idle speed. At this point, the on-coming clutch is engaged to complete the shift. In automotive terms, the shift is referred to as a drive-sync-drive shift since (1) it is initiated at a drive condition where the input speed is lower than the engine speed, (2) it is completed at a synchronous condition where the speed differential across the on-coming clutch is at or near zero, and (3) a drive condition recurs immediately after the shift. It is therefore inherent in this type of control that the coast downshifts occur at relatively low vehicle speeds.